Flash memory commonly uses Fowler-Nordheim tunneling for programming memory cells. The tunneling current is dependent on the voltage gap between the control gate and a channel region. When programming memory cells, program voltage is applied to a wordline containing cells being programmed and other cells, which are to be inhibited from programming. Program disturb occurs when programming of cells causes an inhibited memory cell on the same wordline to become inadvertently programmed.
Program disturb is expected to be a key limiter of NAND memory-cell scaling. As the memory-cell size is scaled down, the natural variation in the cell-to-cell programming behavior increases. As a result, it becomes harder and harder to keep a fast programming cell, which is in an erased state, from disturbing up while ensuring that a slow programming cell can program to a target threshold voltage Vt.
Increasing the channel-boost voltage may help the program disturb problem. Usually a higher channel-boost voltage is obtained by increasing the unselected wordline voltage V_inhibit. An increase in the V_inhibit is, however, limited by what is known as V_inhibit disturb in which cells on a selected string disturb up due to the high V_inhibit voltage.